[nem-en] Macro proposal

[Kamil Dworakowski]

Or maybe this would be more self descriptive and more general.

def sum = function_with_callback(1, 2,_) <| fun(l, r)
    l + r

def sum = function_with_callback(1,_,_) <| 2 <| fun(l, r)
    l + r

Could be implemented as a plain operator(?)